Modularly structured digital communications systems serve the purpose of connecting terminal equipment usually having versatile performance features to one another and for connecting such terminal equipment to communication networks, particularly public networks. A special edition of "telcom report", ISDN im Buero, 1985, discloses such a communications system to which analog and digital telephones, telecopiers, multi-functional terminals, work station systems, personal computers, teletex, picture screen text stations and data terminal equipment can be connected and discloses networking with other communications systems. The essential component part of this communications system is a central communications computer having a system data base and having at least one multi-tasking operating system. A "task" is herein defined as an autonomous, independent execution unit that is composed of a runnable program as well as of its respective run environment, for example memory occupation or apparatus allocation. At every point in time, every task has the status of "active", "waiting" or "quiescent". Tasks can be executed in parallel to one another by a multi-tasking operating system, whereby this can be performed both by means of a plurality of processors working independently of one another as well as by means of a single processor in a time-division multiplex method. Further significant properties of tasks are that they can communicate with one another by status messages and can mutually synchronize one another.
The performance capability of such a digital communications system is critically determined by the software structure of the system. A known prior art communications system has a modularly structured software architecture whose significant component parts are, first, the multi-tasking operating system or the data base and, second, the actual communications software composed of the periphery software, the line technologY software and the switching-oriented connection software. The distribution of sub-functions of the control software to individual software modules or on to the entire software structure of the known communications system is adapted to a system size of several hundred up to several thousand terminals.
The performance and the usefulness of such a communications system essentially depend on the ability to organize and maintain the system that, in addition to other aspects, are subsumed under the term operating technology and depend on the error recognition, error display and error elimination that, in addition to other aspects, are subsumed under the term security technology.
The organizability of the system should be able to handle individual user requests with respect to the system expansion, the numbering plan, the multiplicity of apparatus and communications services in a fast simple reliable and user-friendly way. The most important functions of maintenance are, for example, manual switch requests for inhibiting enabling and switching system units, manual test requests, execution tracking and diagnosis acquisition as well a providing clear text output for the system operator. Prerequisite for the organizability and maintainability of the system is an all-embracing "administration" of the communications system. Belonging to this "administration" are data storage generating and re-generating, storing and caring for the customer data for system configuration, communications services and terminal equipment as well as traffic measurements.
The most important functions of the security technology are test routines of the system hardware components, identification of faults occurring in hardware components or in the control software level of the communications system by monitoring specific system-typical indicators within defined switching-oriented or operations-oriented statuses, and general function monitoring using test switching events and statistical error evaluations.